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This chapter in the NCERT Class 10 Science Book Solutions talks about Magnetic Effects of Electric Current. In this chapter, you will be introduced to Magnetic Fields and Field Lines, and Magnetic Field due to current-carrying conductors. You get to learn about Magnetic Force and Fleming’s Left-Hand and Right-Hand Rules. The chapter also talks about Electric Motors, Electromagnetic Induction, Electric Generators (AC and DC), and Domestic Electric Circuits, including details on Fuses, Overloading, and Short-circuits.
The topics in Chapter 13 include:
With Home Revise, you can be efficiently prepared in all the concepts, applications, and diagrams for your exams.
Chapter 13 - Magnetic Effects of Current Exersice
Q.1 Why does a compass needle get deflected when brought near a bar magnet?.
Ans: A compass needle is a small bar magnet. When it is brought near a bar magnet, its magnetic field lines interact with that of the bar magnet. Hence, a compass needle shows a deflection when brought near a bar magnet.
Q.2 Draw magnetic field lines around a bar magnet.
Ans: Magnetic field lines of a bar magnet emerge from the north pole and terminate at the south pole. Inside the magnet, the field lines emerge from the south pole and terminate at the north pole, as shown in the given figure.
Q.3 List the properties of magnetic lines of force.
Ans:The properties of magnetic lines of force are as follows.
1. Outside a magnet, magnetic field lines are directed from north pole to south pole.
2. The direction of field lines inside the magnet is from the south pole to the north pole.
3. Magnetic lines do not intersect with each other.
4. Magnetic lines of force are crowded near the poles of a magnet but they are widely separated at other places.
Q.4 Why don't two magnetic lines of force intersect each other?
Ans: If two field lines of a magnet intersect, then at the point of intersection, there would be two directions of magnetic field. This is not possible. Hence, two field lines do not intersect each other.
Q.5 Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop.
Ans: Applying right hand thumb rule to the loop:
For right side of the circular loop, the direction of magnetic field lines will be as if they are emerging from the table outside the loop and merging in the table inside the loop. Similarly, for left side of the circular loop, the direction of magnetic field lines will be as if they are emerging from the table outside the loop and merging in the table inside the loop, as shown in the given figure.
Q.6 The magnetic field in a given region is uniform. Draw a diagram to represent it.
Q.7 Choose the correct option.
The magnetic field inside a long straight solenoid-carrying current
(a) is zero
(b) decreases as we move towards its end
(c) increases as we move towards its end
(d) is the same at all points
Ans: (d) is the same at all points
Q.8 Which of the following property of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer.)
Ans: (c) velocity and (d) momentum
When a proton enters the region of magnetic field, it experiences magnetic force. Due to which the path of the proton becomes circular. As a result, the velocity and the momentum change.
Q.9 In Activity 13.7, how do we think the displacement of rod AB will be affected if (i) current in rod AB is increased; (ii) a stronger horse-shoe magnet is used; and (iii) length of the rod AB is increased?
Ans: A current carrying conductor when placed in a magnetic field experiences force. The magnitude of this force will increase with the increase in the amount of current, length of conductor and the strength of the magnetic field. Hence, the strength of the magnetic force exerted on the rod AB and its displacement will increase if
(1) The current in rod AB is increased
(2) Stronger horse shoe magnet is used
(3)When the length of the rod AB increases
Q.10 A positively-charged particle (alpha-particle) projected towards the west is deflected towards north by a magnetic field. The direction of magnetic field is
(1) towards south
(2) towards east
Ans: The direction of the magnetic field can be determined using the Fleming’s Left hand rule. According to the rule, if we arrange our thumb, forefinger and the middle finger of the left hand right perpendicular to each other, then the thumb points towards the direction of the magnetic force, the middle finger the direction of current and the forefinger the direction of magnetic field. Since the direction of positively charged particle is towards west, the direction of the current will also be towards the west. The direction of the magnetic force is towards the north, hence the direction of magnetic field will be upward according to Fleming’s Left hand rule.
Q.11 State Fleming’s left-hand rule.
Ans: According to Fleming's left hand rule, if we arrange the thumb, the central finger, and the forefinger of the left hand at right angles to each other and if the forefinger points in the direction of magnetic field, the central finger points in the direction of current, then the thumb points in the direction of motion of the conductor.
Q.12 What is the principle of an electric motor?
Ans: The working principle of an electric motor is based on the magnetic effect of current. A current-carrying loop experiences a force and rotates when placed in a magnetic field. The direction of rotation of the loop is given by the Fleming's left-hand rule.
Q.13 What is the role of the split ring in an electric motor?
Ans: The split ring in the electric motor acts as a commutator. The commutator reverses the direction of current flowing through the coil after each half rotation of the coil. Due to this reversal of the current, the coil continues to rotate in the same direction.
Q.14 Explain different ways to induce current in a coil.
Ans: The different ways to induce current in a coil are as follows:
1. By relative motion between the coil and a magnet.
2. By relative motion between the coil and a conductor carrying current.
3. By changing the current in a conductor placed near the coil.
Q.15 Explain different ways to induce current in a coil.
Ans: Electric generator works on the principle of electromagnetic induction. In a generator, electricity is generated by rotating a coil in the magnetic field.
Q.16 Name some sources of direct current.
Ans: DC generator and cell are some sources of direct current.
Q.17 Which sources produce alternating current?
Ans: Power plants and AC generators are some of the sources that produce alternating current.
Q.18 Choose the correct option.
A rectangular coil of copper wires is rotated in a magnetic field. The direction of the induced current changes once in each
(a) two revolutions
(b) one revolution
(c) half revolution
(d) one-fourth revolution
Ans: (c) When a rectangular coil of copper is rotated in a magnetic field, the direction of the induced current in the coil changes once in each half revolution. As a result, the direction of current in the coil remains the same.
Q.19 Name two safety measures commonly used in electric circuits and appliances.
Ans: Two safety measures commonly used in electric circuits and appliances are as follows:
(i) Each circuit must be connected with an electric fuse. This prevents the flow of excessive current through the circuit. When the current passing through the wire exceeds the maximum limit of the fuse element, the fuse melts to stop the flow of current through that circuit, hence protecting the appliances connected to the circuit.
(ii) Earthing is a must to prevent electric shocks. Any leakage of current in an electric appliance is transferred to the ground and people using the appliance do not get the shock.
Q.20 An electric oven of 2 kW is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain.
Ans: The current drawn by the electric oven can be calculated using the formula
P = V × I
I = P/V
Substituting the values, we get
I = 2000 W/220 V = 9.09 A
The current drawn by the electric oven is 9.09 A which exceeds the safe limit of the circuit. This causes the fuse to melt and break the circuit.
Q.21 What precaution should be taken to avoid the overloading of domestic electric circuits?
Ans: The precautions that should be taken to avoid the overloading of domestic circuits are as follows:
(a) Too many appliances should not be connected to a single socket.
(b) Too many appliances should not be used at the same time.
(c) Faulty appliances should not be connected in the circuit.
(d) Fuse should be connected in the circuit.
Q.22 Which of the following correctly describes the magnetic field near a long straight wire?
(a) The field consists of straight lines perpendicular to the wire
(b) The field consists of straight lines parallel to the wire
(c) The field consists of radial lines originating from the wire
(d) The field consists of circles centred on the wire
An: d. The field consists of concentric circles centered on the wire.
The magnetic field near a long straight wire are concentric circles. Their centers lie on the wire.
Q.23 The phenomenon of electromagnetic induction is
(a) the process of charging a body
(b) the process of generating magnetic field due to a current passing through a coil
(c) producing induced current in a coil due to relative motion between a magnet and the coil
(d) the process of rotating a coil of an electric motor
Ans: c. producing induced current in a coil due to relative motion between a magnet and the coil.
The phenomenon of inducing current in a coil due to the relative motion between the coil and the magnet Is known as electromagnetic induction.
Q.24 The device used for producing electric current is called a
Ans: a. generator
The device used for producing electric current is known as generator. Generator converts mechanical energy to electric energy.
Q.25 The essential difference between an AC generator and a DC generator is that
(a) AC generator has an electromagnet while a DC generator has permanent magnet.
(b) DC generator will generate a higher voltage.
(c) AC generator will generate a higher voltage.
(d) AC generator has slip rings while the DC generator has a commutator.
Ans: d. AC generator has slip rings while the DC generator has a commutator.
AC generators have two rings known as the slip rings while DC generators have two half rings known as the commutator. This is main difference between AC generator and DC generator.
Q.26 At the time of short circuit, the current in the circuit
(a) reduces substantially
(b) does not change
(c) increases heavily
(d) vary continuously
Ans: (c) increases heavilyWhen two naked wires in the circuit come in contact with each other, the amount of current flowing in the circuit increase abruptly resulting in short circuit.
Q.27 State whether the following statements are true or false.
(a) An electric motor converts mechanical energy into electrical energy.
(b) An electric generator works on the principle of electromagnetic induction.
(c) The field at the centre of a long circular coil carrying current will be parallel straight lines.
(d) A wire with a green insulation is usually the live wire of an electric supply.
An electric motor converts electrical energy into mechanical energy.
An electric generator is a device that generates electricity by rotating a coil in a magnetic field.
A long circular coil is a solenoid. The magnetic field lines inside a solenoid are parallel straight lines.
Live wires have red insulation cover while the earth wire has green insulation.
Q.28 List two methods of producing magnetic fields.
Ans: Two methods of producing magnetic field are as follows:
(a) By using current-carrying conductors
(b) By using permanent magnets
Q.29 How does a solenoid behave like a magnet? Can you determine the north and south poles of a current-carrying solenoid with the help of a bar magnet? Explain.
Ans: A solenoid is a long coil of circular loops of insulated copper wire. The magnetic field produced around the solenoid when the current is passed through it is similar to the magnetic field produced around the bar magnet when current is passed through it. The figure shown below shows the arrangement of magnetic fields produced around the solenoid when current is passed through it.
When the north pole of the bar magnet is brought close to the end connected to the negative terminal of the battery, the solenoid repels the battery. As like poles repel each other, we can infer that the end connected to the negative terminal behaves as a north pole while the end connected to the positive terminal behaves as a south pole.
Q.30 When is the force experienced by a current-carrying conductor placed in a magnetic field largest?
Ans: The force experienced by a current-currying conductor is the maximum when the direction of current is perpendicular to the direction of the magnetic field.
Q.31 Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?
Ans: The direction of magnetic field is given by Fleming's left hand rule. Magnetic field inside the chamber will be perpendicular to the direction of current (opposite to the direction of electron) and direction of deflection/force i.e., either upward or downward. The direction of current is from the front wall to the back wall because negatively charged electrons are moving from back wall to the front wall. The direction of magnetic force is rightward. Hence, using Fleming's left hand rule, it can be concluded that the direction of magnetic field inside the chamber is downward.
Q.32 Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor?
Ans: An electric motor is a device that converts electrical energy to mechanical energy. It works on the principle of magnetic effect of current. The figure listed below shows a simple electric motor.
When a current is allowed to flow through the coil MNST by closing the switch, the coil starts rotating clockwise. This happens because an inward force acts on length MN and at the same time, an outward force acts on length ST. As a result, the coil rotates clockwise.
Current in the length MN flows from M to N and the magnetic field acts from left to right, normal to length MN. Therefore, according to Fleming’s left hand rule, an inward force acts on the length MN. Similarly, current in the length from S to T and the magnetic field acts from left to right, normal to the flow of current. Therefore, an outward force acts on the length ST. These two forces cause the coil to rotate clockwise.
After half a rotation, the position of MN and ST interchange. The half-ring D comes in contact with brush A and half-ring C comes in contact with brush B. Hence, the direction of current in the coil MNST gets reversed.
The current flows through the coil in the direction TSNM. The reversal of current through the coil MNST repeats after each half rotation. As a result, the coil rotates unidirectional. The split rings help to reverse the direction of current in the circuit. These are called the commutator.
Q.33 Name some devices in which electric motors are used?
Ans: Some devices in which electric motors are used are as follows:
(a) Water pumps
(b) Electric fans
(c) Electric mixers
(d) Washing machines
Q.34 A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is (i) pushed into the coil, (ii) withdrawn from inside the coil, (iii) held stationary inside the coil?
Ans: (i) When a bar magnet is pushed into the coil, current is induced in the coil momentarily as a result the galvanometer deflects in a particular direction momentarily.
(ii) When the bar magnet is withdrawn from inside the coil, current is induced momentarily but in the opposite direction and the galvanometer deflects in the opposite direction momentarily.
(iii) When the bar magnet is held stationary inside the coil, no current will be induced as a result there will be no deflection in the galvanometer.
Q.35 Two circular coils A and B are placed closed to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.
Ans: Two circular coils A and B are placed close to each other. When the current in coil A is changed, the magnetic field associated with it also changes. As a result, the magnetic field around coil B also changes. This change in magnetic field lines around coil B induces an electric current in it. This is called electromagnetic induction.
Q.36 State the rule to determine the direction of a
(i) magnetic field produced around a straight conductor-carrying current,
(ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and(iii) current induced in a coil due to its rotation in a magnetic field.
Ans: (i) Maxwell's right hand thumb rule
(ii) Fleming's left hand rule
(iii) Fleming's right hand rule
Q.37 Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?
Ans: The electric generator coverts the mechanical energy into the electrical energy. The working principle of the electric generator is the electromagnetic induction. It generates electricity by rotating a coil in the magnetic field. The figure below shows the construction of a simple AC generator.
In the diagram,
A and B are brushes,
C and D are slip rings
X is the axle
G is the galvanometer
When the axle X is rotated clockwise, MN moves upwards while ST moves downward. The movement of MN and ST in the magnetic field results in the production of electric current due to electromagnetic induction. MN moves upwards and the magnetic fields act from left to right. Therefore, according to Fleming’s right hand rule, the direction of the induced current will be from M to N along the length MN. Similarly, the direction of the induced current will be from S to T along the length ST. The direction of the current in the coil is MNST. Hence, galvanometer shows a deflection in a particular direction.
After half a rotation, length MN starts moving downwards while the length ST starts moving upwards. Now, the direction of the induced current reverses to TSNM. Since the direction of the induced current reverses every half rotation, the current induced is known as alternating current.
Function of Brushes :
Brushes are kept pressed on to two slip rings separately. Outer ends of brushes are connected to the galvanometer. Thus, brushes help in transferring current from coil to the external circuit.
Q.38 When does an electric short circuit occur?
Ans: An electric short circuit occurs when the live wire and the neutral wire come in direct contact. This happens when the plastic insulation of live wire and neutral wire gets torn or when there is a fault in the electrical appliance.
Q.39 What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Ans: The metallic body of electric appliances is connected to the earth by means of earth wire so that any leakage of electric current is transferred to the ground. This prevents any electric shock to the user. That is why earthing of the electrical appliances is necessary.